Re: index prefetching

On 19/01/2024 2:35 pm, Tomas Vondra wrote:
>
> On 1/19/24 09:34, Konstantin Knizhnik wrote:
>> On 18/01/2024 6:00 pm, Tomas Vondra wrote:
>>> On 1/17/24 09:45, Konstantin Knizhnik wrote:
>>>> I have integrated your prefetch patch in Neon and it actually works!
>>>> Moreover, I combined it with prefetch of leaf pages for IOS and it also
>>>> seems to work.
>>>>
>>> Cool! And do you think this is the right design/way to do this?
>> I like the idea of prefetching TIDs in executor.
>>
>> But looking though your patch I have some questions:
>>
>>
>> 1. Why it is necessary to allocate and store all_visible flag in data
>> buffer. Why caller of  IndexPrefetchNext can not look at prefetch field?
>>
>> +        /* store the all_visible flag in the private part of the entry */
>> +        entry->data = palloc(sizeof(bool));
>> +        *(bool *) entry->data = all_visible;
>>
> What you mean by "prefetch field"?

I mean "prefetch" field of IndexPrefetchEntry:

+
+typedef struct IndexPrefetchEntry
+{
+    ItemPointerData tid;
+
+    /* should we prefetch heap page for this TID? */
+    bool        prefetch;
+

You store the same flag twice:

+        /* prefetch only if not all visible */
+        entry->prefetch = !all_visible;
+
+        /* store the all_visible flag in the private part of the entry */
+        entry->data = palloc(sizeof(bool));
+        *(bool *) entry->data = all_visible;

My question was: why do we need to allocate something in entry->data and
store all_visible in it, while we already stored !all-visible in
entry->prefetch.